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2001 VW Passat Intermitent AC

I own a '01 VW 1.8L Passat Wgn w/ 105K mi, well maintained. AC compressor has been replaced once, 4 yrs ago. Currently, sometimes the AC comes on when I start car, sometimes not for for 10 minutes, sometimes not at all. When it comes on, the air is cold, stays cold, and runs continuously (I live in FL). This all happens irrespective of outside temp. It is like there is a relay that is sometimes not closing. My mechanic says he thinks it is low on refrig., but if it were the unit would shut off on high pressure - it does not. He does not know of a relay, and the VW dealer is of no help. Anybody know of a relay I could check, and where/how I find and replace it? Or, any other suggestions?